Animal diversity continues to evolve

Sunday

Oct 30, 2011 at 12:01 AMOct 30, 2011 at 11:40 AM

This term, I am teaching my course on animal diversity, which I last taught a year ago. Lately, I have had to scramble to update my presentations on the main groups of animals and how they differ. This was not always the case.

This term, I am teaching my course on animal diversity, which I last taught a year ago.

Lately, I have had to scramble to update my presentations on the main groups of animals and how they differ. This was not always the case.

This used to be a rather constant subject in biology, but molecular genetics and the insights it provides is changing that.

For centuries, biologists have placed all species into major categories. Aristotle first noted such natural groups in 350 B.C. Carl Linnaeus proposed the first formal system to categorize all living things in 1735.

Today, we use an expanded version of his system and recognize about 35 major groups, or phyla, of animals.

Biologists began asking new questions about these animal groups during the past 50 years.

In particular, why are there 35 animal phyla? Why not 70 or 140? Why is there a cap on the number of animal phyla that can exist on Earth? If complex life exists elsewhere, are there about 35 animal phyla there as well?

Ecologists see living organisms as members of interacting populations, each occupying specific niches consisting of the resources they consume and provide. Ecologists might argue that the environment limits the number of animal phyla to 35.

Developmental biologists see individual plants and animals along timelines that stretch from conception to reproduction. Each stage is determined by the previous one and its genes. For them, the answer to why there are 35 animal phyla is straightforward: There are 35 different ways to make animal embryos.

These perspectives of ecology and developmental biology have converged through molecular genetic studies published in the past decade. Now, we can identify and compare the genes that control development in most animal phyla.

Much to our surprise, a group of just the same few genes controls embryo development in each phylum. The DNA sequence of those genes and even their location along chromosomes in the cell has remained remarkably constant for more than 500 million years.

The key to animal diversity lies in the number of gene copies that control the development of embryos.

Ancient animal phyla, such as jellyfish or flatworms, have few developmental-control genes, while more recent phyla, such as bees, birds and mammals, have many more copies.

The number and complexity of cell types and their associated actions have increased as well. The DNA sequence of these developmental genes and their clustering together along one or a few chromosomes suggest their origin.

Every organism receives a complete set of genetic instructions from its parent. The parent duplicates its genes to start this process. Mutations during duplication can pass to offspring. An increase in the number of the same gene can occur accidentally, like a malfunctioning photocopier.

Natural selection acts on these new copies of perfectly functional genes. When the copied genes control development, the extra copies might lead to variations on the developmental theme of the original.

We have discovered only in the past several years that these mutational increases in the number of developmental genes followed by selection on the resulting new copies occur throughout all of the animal phyla in a consistent, almost predictable, fashion.